1. Field of the Invention
The present invention relates to a cover material, a carrier tape, and a taping apparatus produced by them, and particularly to a carrier tape made of a polycarbonate resin, a cover material used therefor, and a taping apparatus composed of them.
2. Related Background Art
An embossed carrier type of taping apparatus is used in such a structure that electronic parts are housed in embossed portions of a carrier tape having many embossed portions formed therein. The embossed portions of the carrier tape are hermetically sealed by heat-sealing of a cover material (cover tape) onto the carrier tape so as to cover the embossed portions. The carrier tape used for the embossed carrier type of taping apparatus is usually made of a material that is readily formable into a sheet, such as a polyvinyl chloride, a polystyrene, and a polypropylene. Further, the cover material is provided with a biaxially oriented resin film and a heat sealant layer formed on either surface of the film. Since there is such a risk that deterioration or destruction of electronic parts is brought about by static electricity generated due to contact of the housed electronic parts with the embossed portions of the carrier tape or with the cover material and by static electricity generated when the cover material is peeled off from the carrier tape, the carrier tape and the cover material need to include means for preventing it.
In order to prevent generation of static electricity in the carrier tape, means normally employed were mixing electrically conductive carbon fine particles or metal fine particles in the carrier tape or applying such particles onto the carrier tape. In order to prevent generation of static electricity in the cover material, means normally employed were mixing an antistatic agent such as a surfactant, electrically conductive carbon fine particles, or metal fine particles in the heat sealant layer in direct contact with the electronic parts or applying either of them onto the heat sealant layer.
However, the conventional embossed carrier type of taping apparatus as described above had a problem that the carrier tape and the cover material had extremely low transparency because of the electrically conductive carbon fine particles contained therein as an antistatic agent, which made it difficult to check the electronic parts housed in the embossed carrier type of taping apparatus from the outside.
Further, the coating of the surfactant had a problem that it changed a surface condition of the heat sealant layer of the cover material to make the sealing property of the heat sealant layer unstable, thus resulting in a failure of sealing, and a problem that a stable antistatic effect was not obtained because of a strong dependence of electrostatic diffusion on the temperature or the humidity during storage.
Furthermore, the heat-sealing of the cover material to the carrier tape was required to yield such predetermined strength that the electronic parts could be prevented from dropping out because of separation of the cover material during transportation and storage of the embossed carrier type taping. However, too high heat-sealing strength raised another problem of causing such an accident that when the cover material was peeled off in a packaging process of electronic parts, vibration of the carrier tape caused the electronic parts to spring out from the embossed portions of the carrier tape. Accordingly, the cover material was required to adhere to the carrier tape with sufficient strength and to have good peelability in use of the electronic parts. This raised a problem that it was hard to determine conditions such as the temperature, the time, or the like of heat-sealing.
Proposed as a means to solve the above problems was a cover material having an intermediate layer, which was formed between the biaxially oriented resin film and the heat sealant layer forming the cover material and which was made of either one polyolefin selected from a polyethylene, an ethylene-vinyl acetate copolymer, an ionomer, a polypropylene, and modified products of the foregoing (Japanese Laid-open Patent Application No. 5-8339). This cover material is peelable at the interface between the intermediate layer and the heat sealant layer in peeling-off, but has a problem that it is not easy to control peel strength (adhesion) between the intermediate layer and the heat sealant layer.
Further, the resins used for the carrier tape, such as a polyvinyl chloride, a polystyrene, and a polypropylene were inferior in continuous heat resistance because of properties of the resins per se. Thus, if these resins were used for the carrier tape to be formed from a resin sheet selected therefrom and if the carrier tape was stored at high temperature (for example, in a shipment condition at about 60.degree. C.), problems arose, for example such as deformation of formed portions (embossed portions), deviation of pitch of feed holes (sprocket holes), etc. Further, in shaping or forming the carrier tape, the embossed portions were normally formed by shaping a pre-heated portion of the above resin sheet by a metal mold. However, shrinkage or expansion of the resin sheet occurred in the pre-heating, and steps to determine the dimensions of the mold for forming the feed holes (sprocket holes) and the embossed portions with high dimensional accuracy, taking such shrinkage or expansion into account, were very complicated. Furthermore, there was another problem that if a non-stretched portion existed due to unevenness of the pre-heating, a hole could be made in this portion.